1. Technical Field of the Invention
This invention relates to optical lithography and more particularly to direct write optical lithography.
2. Description of the Related Art
Polymer arrays, such as the GeneChip.RTM. probe array (Affymetrix, Inc., Santa Clara, Calif.), can be synthesized using light-directed methods described, for example, in U.S. Pat. Nos. 5,143,854; 5,424,186; 5,510,270; 5,800,992; 5,445,934; 5,744,305; 5,384,261 and 5,677,195 and PCT published application no. WO 95/11995, which are hereby incorporated by reference in their entireties. As an example, light-directed synthesis of oligonucleotides employs 5'-protected nucleosidephosphoramidite "building blocks." The 5'-protecting groups may be either photolabile or acid-labile. A plurality of polymer sequences in predefined regions are synthesized by repeated cycles of deprotection (selective removal of the protective group) and coupling. Coupling (i.e., nucleotide or monomer addition) occurs only at sites that have been deprotected. Three methods of light-directed synthesis are: use of photolabile protecting groups and direct photodeprotection (DPD); use of acid-labile 4,4'-dimethoxytrityl (DMT) protecting groups and a photoresist; use of DMT protecting groups and a polymer film that contains a photoacid generator (PAG).
These methods have many process steps similar to those used in semiconductor integrated circuit manufacturing. These methods also often involve the use of photomasks (masks) that have a predefined image pattern which permits the light used for synthesis of the polymer arrays to reach certain regions of a substrate but not others. The substrate can be non-porous, rigid, semi-rigid,etc. It can be formed into a well, a trench, a flat surface, etc. The substrate can include solids, such as siliceous substances such as silicon, glass, fused silica, quartz, and other solids such as plastics and polymers, such as polyacrylamide, polystyrene, polycarbonate, etc. Typically, the solid substrate is called a wafer from which individual chips are made (See the U.S. patents above which are incorporated herein by reference). As such, the pattern formed on the mask is projected onto the wafer to define which portions of the wafer are to be deprotected and which regions remain protected. See, for example, U.S. Pat. Nos. 5,593,839 and 5,571,639 which are hereby incorporated by reference in their entireties.
The lithographic or photochemical steps in the synthesis of nucleic acid arrays may be performed by contact printing or proximity printing using photomasks. For example, an emulsion or chrome-on-glass mask is placed in contact with the wafer, or nearly in contact with the wafer, and the wafer is illuminated through the mask by light having an appropriate wavelength. However, masks can be costly to make and use and are capable of being damaged or lost.
In many cases a different mask having a particular predetermined image pattern is used for each separate photomasking step, and synthesis of a wafer containing many chips requires a plurality of photomasking steps with different image patterns. For example, synthesis of an array of 20mers typically requires approximately seventy photolithographic steps and related unique photomasks. So, using present photolithographic systems and methods, a plurality of different image pattern masks must be pre-generated and changed in the photolithographic system at each photomasking step. This plurality of different pattern masks adds lead time to the process and complexity and inefficiency to the photolithographic system and method. Further, contact printing using a mask can cause defects on the wafer so that some of the reaction sites are rendered defective. Thus, aphotolithographic system and method that does not require such masks and obviates such difficulties would be generally useful in providing a more efficient and simplified lithographic process.